trigger TaskRollUpOnContact on Task (after insert, after update, after delete, after undelete) {
	list<Task> listTask = new list<Task>() ;
    set<Id> setContactWhoId = new set<Id>() ;
    if(Trigger.isInsert){
        for(Task oTask : Trigger.New){
        	if(oTask.Status == 'Completed') setContactWhoId.add(oTask.WhoId) ;
    	}
    }
    system.debug('after insert-setContactWhoId:: ' + setContactWhoId) ;
    if(Trigger.isUpdate){
        for(Task oTask : Trigger.New){
            if(oTask.Status == 'Completed' && oTask.Status != Trigger.oldMap.get(oTask.Id).Status) 
                setContactWhoId.add(oTask.WhoId) ;
        }
    }
    system.debug('after update-setContactWhoId:: ' + setContactWhoId) ;
    if(Trigger.isDelete){
        for(Task oTask : Trigger.Old){
            if(oTask.Status == 'Completed') 
                setContactWhoId.add(oTask.WhoId) ;
        }
    }
    system.debug('after delete-setContactWhoId:: ' + setContactWhoId) ;
    if(Trigger.isUndelete){
        for(Task oTask : Trigger.New){
            if(oTask.Status == 'Completed') 
                setContactWhoId.add(oTask.WhoId) ;
        }
    }
    system.debug('after undelete-setContactWhoId:: ' + setContactWhoId) ;
    if(!setContactWhoId.isEmpty()){
        list<Contact> listContactToUpdate = new list<Contact>() ;
        list<Contact> listContact = [Select Id, No_of_Completed_Task__c, (Select Id From Tasks Where Status = 'Completed') From Contact Where Id IN: setContactWhoId] ;
        if(listContact != null && !listContact.isEmpty()){
            for(Contact oContact : listContact){
                oContact.No_of_Completed_Task__c = oContact.Tasks.size() ;
                listContactToUpdate.add(oContact) ;
            }
        }
        if(!listContactToUpdate.isEmpty()) update listContactToUpdate ;
    }
}